output

{
  "a": {
    "b": {
      "c": {
        "d": 43
      }
    }
  },
  "f": "hello"
}

src

void Main()
{
	dynamic thing = new Expandable();
	
	thing.a.b.c.d = 43;
	thing.f = "hello";
	
	string json = JsonConvert.SerializeObject(thing, Newtonsoft.Json.Formatting.Indented);
	json.Dump();
}

// Define other methods, classes and namespaces here

public class Expandable : DynamicObject
{
	Dictionary<string, object> _members = new Dictionary<string, object>();

	public override bool TryGetMember(GetMemberBinder binder, out object result)
	{
		if (!_members.ContainsKey(binder.Name))
			_members.Add(binder.Name, new Expandable());

		result = _members[binder.Name];

		return true;
	}

	public override IEnumerable<string> GetDynamicMemberNames()
	{
		return _members.Keys;
	}

	public override bool TrySetMember(SetMemberBinder binder, object value)
	{
		_members[binder.Name] = value;
		return true;
	}
}